This vegetable is a member of the turnip family and, for that reason, is also called cabbage turnip. Like the turnip, both its purple-tinged, white bulblike stem and its greens are edible. The kohlrabi bulb tastes like a mild, sweet turnip. It's available from midspring to midfall. Those under 3 inches in diameter are the most tender. Choose a kohlrabi that is heavy for its size with firm, deeply colored green leaves. Avoid any with soft spots on the bulb or signs of yellowing on leaf tips. Store tightly wrapped up to 4 days in the refrigerator. Kohlrabi's best steamed, but can also be added to soups and stews as well as used in stir-fries. It's rich in potassium and vitamin C.